Learn Hardcore Swedish: Ju tystare vi pratar, desto lugnare blir barnen. - The quieter we speak, the calmer the children become.

The ju‑clause behaves like a subordinate clause in Swedish, which uses subject–verb order: vi pratar (not verb‑second).
The second half is a main clause and must be verb‑second (V2), so with an initial desto … phrase the verb comes next: desto lugnare blir barnen (not desto lugnare barnen blir).

Do I need the comma?

Yes. In writing, put a comma between the ju‑clause and the desto‑clause:

Ju … , desto …
A semicolon or dash is unnecessary; the comma is the standard choice.

Does ju here mean the modal particle (as in “you know/after all”)?

No. This ju is part of the correlative pair ju … desto … and means something like English “the (more/less).”
The modal particle ju is different, e.g. Det är ju kallt idag (a stance marker meaning “as you know/obviously”).

Are tystare and lugnare adjectives or adverbs here?

tystare functions adverbially (modifies the verb pratar). In Swedish, many adverbs look like the adjective; there’s no special ending.
lugnare is a predicative adjective describing barnen. Comparatives in Swedish don’t inflect for gender/number/definiteness: tystare, lugnare stay the same.

Can I say mer tyst or mer lugn instead of tystare and lugnare?

Prefer the -are forms here:

tystare (not mer tyst)
lugnare (not mer lugn) Use mer with many longer adjectives/adverbs: mer intressant, mer försiktigt. Don’t mix forms like mer tystare.

Why is it blir barnen and not är barnen?

blir highlights change/result: the children become calmer as a consequence.
är would present a static correlation of states (they are calmer when we speak more quietly). It’s possible, but blir is the natural choice when talking about an effect.

Can I use talar instead of pratar?

Yes. tala is a bit more formal; prata is neutral/colloquial. Both work:

Ju tystare vi talar, desto …
Ju tystare vi pratar, desto …

Why barnen and not just barn?

barnen is the definite plural of barn (ett barn → barn; barnen = the children).
Use barnen when you’re talking about specific, context‑known children (e.g., the kids present). Using bare barn would sound like a general statement about children in general and is less natural here.

Can I reverse the order of the clauses?

Yes:

Desto lugnare blir barnen, ju tystare vi pratar. Keep the comma, and remember V2 in the main clause still applies: after desto … the verb (blir) comes before the subject (barnen).

Do I ever need som in these structures?

Yes, when the comparative phrase itself is a head that needs a clause, especially with quantities:

Ju fler som kommer, desto roligare blir det.
Here som kommer modifies fler. In your sentence, tystare directly modifies pratar, so no som is needed.

How would I express the negative idea?

Use a negative comparative instead of inte:

Ju mindre vi pratar, desto lugnare blir barnen.
Ju mindre oväsen vi gör, desto lugnare blir barnen. Avoid forms like ju tystare vi inte pratar.

Is it ever ju … ju … or ju … så … instead of ju … desto …?

In modern standard Swedish, use ju … desto …. You may see dess in some set phrases (ju förr, dess bättre), but desto is the default today. Avoid ju … så … in this construction.

Any quick pronunciation tips?

ju: front rounded vowel, like a long y‑sound.
tystare: short y (like German ü), double s‑length on the first s.
lugnare: the gn is pronounced [ŋn] (ng‑n).
desto: clear double s sound in the middle.
barnen: the rn typically becomes a retroflex nasal [ɳ] in many accents.
Natural speech places a pause after the ju‑clause (matching the comma).